This invention relates to measuring apparatus and, more particularly, to measuring apparatus that can be used to determine voltage waveforms on metalization lines of any integrated circuit or electronic device.
Integrated circuits are becoming larger and larger in size with several hundred thousand transistors per circuit. The number of transistors per circuit and their speed of operation has been increasing to the point where mechanical probes are no longer usable to sample the waveforms present on these circuits. One technique that has evolved to replace the mechanical probe is known as electron beam probing. This technique uses an electron beam of the type generated in scanning electron microscopes to impinge on a metal line of the circuit. The emission of secondary electrons from the integrated circuit line is then utilized in order to determine the voltage present during a given instant in time. Insertion of a delay in either the voltage waveform or in the electron beam can be utilized to determine, in a stroboscopic fashion, the value of the voltage waveform during several instants in time thereby completing the voltage waveform measurement. A description of this type of electron beam probing can be found in the article entitled "Fundamentals of Electron Beam Testing of Integrated Circuits," by E. Menzel et al., Scanning, Vol. 5, 1983, pp. 103-122. Electron beam probing systems are now commercially available and are capable of waveform sampling with 200 picosecond time resolution. Unfortunately the integrated circuit art is proceeding at a rapid pace in producing faster and faster circuits such that this time resolution will no longer be satisfactory. Even greater resolutions in the order of tens of picoseconds will soon be necessary.